Interference mitigation for downlink in a wireless communication system

ABSTRACT

Techniques for mitigating interference in a wireless communication system are described. In an aspect, pertinent transmission parameters for a served UE may be sent to at least one interfered UE to support interference mitigation. In one design, information for at least one transmission parameter for a data transmission sent by a first cell to a first UE may be transmitted to at least one UE served by a second cell to enable the at least one UE to perform interference mitigation for the data transmission sent by the first cell to the first UE. The information may be transmitted by either the first cell or the second cell. In another aspect, a cell may send transmission parameters for a UE via a pilot. In yet another aspect, scrambling may be performed by a cell at symbol level to enable an interfered UE to distinguish between modulation symbols of desired and interfering transmissions.

The present application is a continuation of U.S. application Ser. No.12/763,836, entitled “INTERFERENCE MITIGATION FOR DOWNLINK IN A WIRELESSCOMMUNICATION SYSTEM,” filed Apr. 20, 2010, which claims priority toprovisional U.S. Application Ser. No. 61/184,206, entitled “ENABLINGDOWNLINK INTERFERENCE MITIGATION,” filed Jun. 4, 2009, and Ser. No.61/184,670, entitled “ENABLING DOWNLINK INTERFERENCE MITIGATION,” filedJun. 5, 2009, both assigned to the assignee hereof and incorporatedherein by reference.

BACKGROUND

I. Field

The present disclosure relates generally to communication, and morespecifically to techniques for mitigating interference in a wirelesscommunication system.

II. Background

Wireless communication systems are widely deployed to provide variouscommunication content such as voice, video, packet data, messaging,broadcast, etc. These wireless systems may be multiple-access systemscapable of supporting multiple users by sharing the available systemresources. Examples of such multiple-access systems include CodeDivision Multiple Access (CDMA) systems, Time Division Multiple Access(TDMA) systems, Frequency Division Multiple Access (FDMA) systems,Orthogonal FDMA (OFDMA) systems, and Single-Carrier FDMA (SC-FDMA)systems.

A wireless communication system may include a number of base stationsthat can support communication for a number of user equipments (UEs). AUE may communicate with a base station via the downlink and uplink. Thedownlink (or forward link) refers to the communication link from thebase station to the UE, and the uplink (or reverse link) refers to thecommunication link from the UE to the base station.

A UE may communicate with a serving cell and may be within range of oneor more neighbor cells. The term “cell” can refer to a coverage area ofa base station and/or a base station subsystem serving the coveragearea. The UE may receive a data transmission sent by the serving cell tothe UE on the downlink. The UE may also receive other data transmissionssent by the neighbor cells to other UEs. These other data transmissionsmay appear as interference to the UE and may impact the ability of theUE to recover the data transmission from the serving cell. It may bedesirable to mitigate the interference on the downlink in order toimprove performance.

SUMMARY

Techniques for mitigating interference in a wireless communicationsystem are described herein. In an aspect, pertinent transmissionparameters for a served UE may be sent to at least one interfered UE toenable each interfered UE to perform interference mitigation. Thetransmission parameters may include a modulation order or modulationscheme, a traffic-to-pilot ratio (T2P), precoding information, atransmission rank, downlink resources, and/or other parameters for adata transmission to the served UE.

In one design, information for at least one transmission parameter for adata transmission sent by a first cell to a first UE may be obtained.The information for the at least one transmission parameter may betransmitted to at least one UE served by a second cell to enable the atleast one UE to perform interference mitigation for the datatransmission sent by the first cell to the first UE. The information maybe obtained and transmitted by either the first cell or the second cell.

In one design, the first UE may obtain a received signal comprising afirst data transmission sent by the first cell to the first UE and asecond data transmission sent by the second cell to a second UE. Thefirst UE may also obtain information for the at least one transmissionparameter for the second data transmission, e.g., from the first cell orthe second cell. The first UE may perform interference mitigation forthe second data transmission based on the information for the at leastone transmission parameter to recover the first data transmission sentto the first UE.

In another aspect, a cell may send transmission parameters for a UE viaa pilot sent to the UE. In one design, the cell may generate a datatransmission based on at least one transmission parameter. The cell mayalso generate a pilot (e.g., a dedicated pilot or a UE-specificreference signal) comprising information for the at least onetransmission parameter. In one design, the cell may generate apseudo-random number (PN) sequence based on the information for the atleast one transmission parameter and may then generate modulationsymbols for the pilot based on the PN sequence. The cell may transmitthe pilot and the data transmission to a recipient UE. Other UEs may usethe information for the at least one transmission parameter in the pilotto perform interference mitigation for the data transmission.

In yet another aspect, scrambling may be performed by a cell at symbollevel to enable an interfered UE to distinguish between modulationsymbols of a desired transmission and modulation symbols of aninterfering transmission. In one design, a cell may generate modulationsymbols for a data transmission for a recipient UE and may scramble themodulation symbols based on a scrambling sequence to obtain scrambledsymbols. The cell may generate the scrambling sequence based on a cellidentity (ID) and/or a UE ID. The cell may transmit the scrambledsymbols for the data transmission. The recipient and interfered UEs mayeach perform descrambling with its scrambling sequence and may be ableto distinguish between the modulation symbols of its data transmissionand the modulation symbols of interfering transmissions, which may beuseful for interference mitigation.

Various aspects and features of the disclosure are described in furtherdetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system.

FIG. 2 shows a transmit processor that can generate a dedicated pilotcarrying transmission parameters for data transmission for a UE.

FIG. 3 shows a transmit processor that can perform symbol-levelscrambling.

FIG. 4 shows a receive processor that can perform symbol-leveldescrambling.

FIGS. 5 and 6 show a process and an apparatus, respectively, forperforming interference mitigation.

FIGS. 7 and 8 show a process and an apparatus, respectively, for sendingtransmission parameters to support interference mitigation.

FIGS. 9 and 10 show a process and an apparatus, respectively, forsending transmission parameters via a pilot.

FIGS. 11 and 12 show a process and an apparatus, respectively, forperforming symbol level scrambling.

FIGS. 13 and 14 show a process and an apparatus, respectively, forperforming symbol level descrambling.

FIG. 15 shows a block diagram of a base station and a UE.

DETAILED DESCRIPTION

The techniques described herein may be used for various wirelesscommunication systems such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA and othersystems. The terms “system” and “network” are often usedinterchangeably. A CDMA system may implement a radio technology such asUniversal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. cdma2000 coversIS-2000, IS-95 and IS-856 standards. A TDMA system may implement a radiotechnology such as Global System for Mobile Communications (GSM). AnOFDMA system may implement a radio technology such as Evolved UTRA(E-UTRA), Ultra Mobile Broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, Flash-OFDM®, etc. UTRA and E-UTRA are part ofUniversal Mobile Telecommunication System (UMTS). 3GPP Long TermEvolution (LTE) and LTE-Advanced (LTE-A) are new releases of UMTS thatuse E-UTRA, which employs OFDMA on the downlink and SC-FDMA on theuplink. UTRA, E-UTRA, UMTS, LTE, LTE-A and GSM are described indocuments from an organization named “3rd Generation PartnershipProject” (3GPP). cdma2000 and UMB are described in documents from anorganization named “3rd Generation Partnership Project 2” (3GPP2). Thetechniques described herein may be used for the systems and radiotechnologies mentioned above as well as other systems and radiotechnologies.

FIG. 1 shows a wireless communication system 100, which may be an LTEsystem, a CDMA system, etc. System 100 may include a number of basestations and other network entities. For simplicity, only two basestations 110 and 112 are shown in FIG. 1. A base station may be anentity that communicates with UEs and may also be referred to as a NodeB, an evolved Node B (eNB), an access point, etc. Each base station mayprovide communication coverage for a particular geographic area. Toimprove system capacity, the overall coverage area of a base station maybe partitioned into multiple (e.g., three) smaller areas. In 3GPP, theterm “cell” can refer to the smallest coverage area of a base stationand/or a base station subsystem serving this coverage area. The basestations may communicate with other network entities (e.g., other basestations and/or network controllers) via a backhaul.

A number of UEs may be dispersed throughout the system, and each UE maybe stationary or mobile. For simplicity, only two UEs 120 and 122 areshown in FIG. 1. A UE may also be referred to as a mobile station, aterminal, an access terminal, a subscriber unit, a station, etc. A UEmay be a cellular phone, a personal digital assistant (PDA), a wirelessmodem, a wireless communication device, a handheld device, a laptopcomputer, a cordless phone, a wireless local loop (WLL) station, a smartphone, a netbook, a smartbook, etc. A UE may communicate with a servingcell and may be within range of one or more neighbor cells. The UE mayreceive a desired transmission from the serving cell and may alsoreceive interfering transmissions from the neighbor cells on thedownlink.

In the description herein, the terms “cell” and “base station” may beused interchangeably. A serving cell is a cell or base stationdesignated to serve a UE on the downlink. A neighbor cell is a cell orbase station not serving a UE. The terms “transmission” and “signal” maybe used interchangeably.

In FIG. 1, UE 120 may communicate with its serving cell 110, and UE 122may communicate with its serving cell 112. UE 120 may receive a desiredtransmission from serving cell 110 as well as interfering transmissionsfrom neighbor cell 112. The interfering transmissions may be intendedfor UE 122 and/or other UEs served by cell 112. UE 120 may thus be aninterfered UE for the transmissions from cell 112 to UE 122 and otherUEs. In general, UE 120 may receive any number of interferingtransmissions from any number of neighbor cells. For simplicity, much ofthe description below assumes the example shown in FIG. 1, with oneinterfering transmission from one neighbor cell 112 to one UE 122.

Interference mitigation may be performed by a UE to mitigateinterference on the downlink from neighbor cells. Interferencemitigation refers to a process to address (e.g., suppress) interferencein a received signal in order to improve the likelihood of recovering adesired transmission in the received signal. Interference mitigation maybe accomplished via interference cancellation. Interference mitigationdeals with interference that is present in a received signal whereasinterference avoidance attempts to completely avoid interference, e.g.,by sending transmissions on different frequency regions and/or indifferent time intervals. Interference mitigation may be used to improvesystem capacity, extend coverage, and/or improve data transmissionperformance of UEs that are exposed to strong interfering cells.

Interference mitigation may be categorized into two main classes:

-   -   Packet-level interference mitigation—mitigate interference by        exploiting the code structure of an interfering transmission,        and    -   Symbol-level interference mitigation—mitigate interference based        on knowledge or assumption of a modulation order of an        interfering transmission.

An example of packet-level interference mitigation is post-decodinginterference cancellation, which may be performed as follows. UE 120 mayobtain a received signal comprising a desired transmission from servingcell 110 and an interfering transmission from neighbor cell 112, whichmay be intended for UE 122. UE 110 may process the received signal anddecode the interfering transmission in the received signal. If theinterfering transmission is decoded successfully, then UE 120 mayestimate the interference due to the interfering transmission based onthe decoded data and may then subtract the estimated interference fromthe received signal to obtain an interference-canceled signal having ahigher signal-to-noise-and-interference ratio (SINR). UE 120 may thenprocess the interference-canceled signal and decode the desiredtransmission. Data transmission performance may improve (e.g., a higherdata rate may be supported) by the higher SINR obtained withinterference cancellation.

Post-decoding interference cancellation may be supported if UE 120 canobtain pertinent transmission parameters to receive and decode theinterfering transmission. These parameters may include a modulation andcoding scheme (MCS) used for the interfering transmission, downlinkresources (e.g., resource blocks) on which the interfering transmissionis sent, etc. Neighbor cell 112 may send control information (e.g., adownlink grant) comprising these parameters to UE 122. Neighbor cell 112may send the downlink grant with power control and/or rate control sothat the downlink grant can be reliably received by intended UE 122.Hence, UE 120 may not be able to decode the downlink grant sent to UE122 with power and/or rate control.

Furthermore, even if UE 120 can obtain the pertinent transmissionparameters for the interfering transmission, UE 120 may be unable tosuccessfully decode the interfering transmission. The MCS for theinterfering transmission may be selected based on the channel qualitybetween neighbor cell 112 and UE 122. The channel quality betweenneighbor cell 112 to its UE 122 will likely be better than the channelquality between neighbor cell 112 and UE 120. This may be due to thefact that each UE typically associates with the strongest cell, whichmay be serving cell 110 for UE 120. Consequently, UE 120 may receive theinterfering transmission from neighbor cell 112 with a lower SINR thanthe required SINR for the MCS used for the interfering transmission.

Symbol-level interference mitigation may be performed when packet-levelinterference mitigation is not practical or when lower complexity isdesired. Symbol-level interference mitigation does not require UE 120 tocorrectly decode an interfering transmission. Instead, UE 120 maydemodulate the interfering transmission and may estimate and cancel theinterference due to the modulation symbols. Symbol-level interferencemitigation may be categorized into two main classes:

-   -   Soft-symbol interference cancellation—modulation symbols of an        interfering transmission are estimated and subtracted from a        received signal to improve SINR, and    -   Joint demodulation—modulation symbols of an interfering        transmission and a desired transmission are jointly demodulated,        which may lead to calculation of “soft bits” or log-likelihood        ratios (LLRs) of the desired transmission to be applied to a        decoder for the desired transmission.

Soft-symbol interference cancellation or joint demodulation may also beperformed iteratively with decoding. In this case, demodulation withsoft-symbol interference cancellation or joint demodulation may beperformed on received symbol to obtain demodulated symbols, thendecoding may be performed on the demodulated symbols to obtain decoderoutput, and the processing may be repeated one or more times with theoutput of the demodulation step being provided to the decoding step, andvice versa. The demodulation step may thus alternate with the decodingstep to obtain better estimates of modulation symbols of the desiredtransmission

In an aspect, pertinent transmission parameters for a data transmissionfor a served UE may be sent to at least one interfered UE to enable eachinterfered UE to perform interference mitigation. Symbol-levelinterference mitigation may be supported by providing the interferedUE(s) with pertinent transmission parameters to demodulate aninterfering transmission and to estimate interference due to thesemodulation symbols. These parameters may include one or more of thefollowing:

Modulation order/modulation scheme used for the interferingtransmission,

T2P of the interfering transmission,

Precoding information for the interfering transmission,

Transmission rank of the interfering transmission,

Downlink resources used for the interfering transmission, and/or

Other transmission parameters.

UE 120 may demodulate the interfering transmission based on themodulation order used for the interfering transmission. This modulationorder may be selected from a set of modulation orders supported by thesystem. The supported modulation orders may include binary phase shiftkeying (BPSK), quadrature phase shift keying (QPSK), 16-level quadratureamplitude modulation (16-QAM), 64-QAM, etc. In one design, UE 120 mayobtain the modulation order (rather than the MCS) of the interferingtransmission as described below and may perform demodulation based onthe modulation order. In another design, UE 120 may assume a particularmodulation order (e.g., QPSK) for the interfering transmission and mayperform demodulation based on the assumed modulation order. UE 120 mayachieve maximum gain when the actual modulation order matches theassumed modulation order and may obtain substantial gain even when theactual modulation order (e.g., 16-QAM or 64-QAM) does not match theassumed modulation order (e.g., QPSK). In yet another design, UE 120 mayperform demodulation based on multiple hypotheses for modulation orderand may select a hypothesis associated with the highest likelihood ofcorrect demodulation. The best hypothesis may be identified based on oneor more metrics computed for each hypothesis, such as absolute values ofLLRs in the case of joint demodulation, post-cancellation SINR in thecase of demodulation with interference cancellation, etc.

UE 120 may estimate the wireless channel from neighbor cell 112 to UE120 and may use the channel estimate to perform interference mitigation.UE 120 may perform channel estimation in different manners depending onhow a reference signal (or pilot) and data are transmitted by neighborcell 112. UE 120 may use the T2P and the precoding information for theinterfering transmission to derive the channel estimate, as describedbelow. The precoding information may convey weights used for precodingby neighbor cell 112 for UE 122. The precoding information may comprisea precoding matrix indicator (PMI), a transmission mode indicator, etc.The PMI may indicate a specific vector of weights used for precoding.The RI may indicate the number of data streams or packets to transmit.The transmission mode indicator may indicate whether closed-loopprecoding or open-loop precoding, such as large delay cyclic delaydiversity (LD-CDD) in LTE, is used. Closed-loop precoding or open-loopprecoding may be performed based on specified precoding weights.

UE 120 may demodulate the interfering transmission based further on thetransmission rank of the interfering transmission. The transmission rankmay indicate the number of data streams transmitted by neighbor cell 112to UE 122. UE 120 may also demodulate the interfering transmission basedon the downlink resources used for the interfering transmission.

The received signal at UE 120 may be expressed as:X(k)=H ₁ ·S ₁(k)+H ₂ ·S ₂(k)+N,  Eq (1)where S₁(k) is a desired transmission from serving cell 110 to UE 120,

S₂(k) is an interfering transmission from neighbor cell 112,

H₁ is a channel gain from serving cell 110 to UE 120,

H₂ is a channel gain from neighbor cell 112 to UE 120,

X(k) is the received signal at UE 120, and

N is additive noise at UE 120.

In equation (1), k may be an index for downlink resources used to sendthe desired and interfering transmissions. For simplicity, equation (1)assumes a flat fading channel with a constant channel gain for alldownlink resources. The channel gain may also be a function of resourceindex k. Also for simplicity, equation (1) assumes only one interferingtransmission from one neighbor cell.

UE 120 may perform minimum mean square error (MMSE) demodulation on thereceived signal by treating the interfering transmission asinterference, as follows:

$\begin{matrix}{{{{\hat{S}}_{1}(k)} = \frac{{X(k)} \cdot {\hat{H}}_{1}^{*}}{{{\hat{H}}_{1}}^{2} + \sigma_{n\; 1}^{2}}},} & {{Eq}\mspace{14mu}(2)}\end{matrix}$where Ĥ₁ is an estimate of the channel gain from serving cell 110 to UE120,

-   -   Ŝ₁ (k) is an estimate of the desired transmission,    -   σ_(n1) ² is a variance of total noise and interference for the        desired transmission, which is H₂·S₂ (k)+N, and    -   “*” denotes a complex conjugate.

For simplicity, equation (2) assumes one receive antenna at UE 120.Multiple-input multiple-output (MIMO) detection based on MMSE may alsobe performed if UE 120 is equipped with multiple receive antennas. Sincethe interfering transmission is treated as interference in equation (2),UE 120 may obtain a lower SINR for the desired transmission.

In one design, UE 120 may perform soft-symbol interference cancellationto improve performance. For soft-symbol interference cancellation, UE120 may first perform demodulation to obtain an estimate of theinterfering transmission based on the following criterion:Ŝ ₂(k)=E{S ₂(k)|X(k),Ĥ ₁ ,Ĥ ₂,σ_(n) ² ,Q ₂},  Eq (3)where Ĥ₂ is an estimate of the channel gain from neighbor cell 112 to UE120,

Ŝ₂(k) is an estimate of the interfering transmission, and

Q₂ denotes a modulation order of the interfering transmission assumed byUE 120,

σ_(n) ² is a variance of additive noise N in equation (1), and

E{U|V} denotes an expected value of U when V is observed.

As shown in equation (3), UE 120 may obtain an estimate Ŝ₂(k) of theinterfering transmission based on the received signal X(k), the channelestimates Ĥ₁ and Ĥ₂, the noise variance σ_(n) ², and the modulationorder Q₂ of the interfering transmission.

UE 120 may then estimate and subtract the interference due to theinterfering transmission from the received signal, as follows:Y(k)=X(k)−Ĥ ₂ ·Ŝ ₂(k)=H ₁ ·S ₁(k)+N _(r),  Eq (4)where Ĥ₂·Ŝ₂ (k) is estimated interference due to the interferingtransmission,

Y(k) is an interference-canceled signal, and

N_(r) is residual noise and interference for the desired transmission.

UE 120 may then perform demodulation on the interference-canceledsignal, as follows:

$\begin{matrix}{{{{\hat{S}}_{1}(k)} = \frac{{Y(k)} \cdot {\hat{H}}_{1}^{*}}{{{\hat{H}}_{1}}^{2} + \sigma_{nr}^{2}}},} & {{Eq}\mspace{14mu}(5)}\end{matrix}$where σ_(nr) ² is a variance of N_(r).

Since the interfering transmission is removed in theinterference-canceled signal, UE 120 may obtain a higher SINR for thedesired transmission due to soft-symbol interference cancellation. UE120 may further process (e.g., decode) the demodulated symbols for thedesired transmission to recover data sent by serving cell 110 to UE 120.

In another design, UE 120 may perform joint demodulation to improveperformance. UE 120 may estimate the wireless channels for serving cell110 and neighbor cell 112. UE 120 may then perform joint demodulationbased on maximum a posteriori probability (MAP) criterion, which may beequivalent to minimizing a distance metric in some cases. The MAPcriterion may be expressed as follows:

$\begin{matrix}{{{M\left( {{{\hat{S}}_{1}(k)},{{\hat{S}}_{2}(k)}} \right)} = {- \frac{{{{X(k)} - {{\hat{H}}_{1} \cdot {{\hat{S}}_{1}(k)}} - {{\hat{H}}_{2} \cdot {{\hat{S}}_{2}(k)}}}}^{2}}{\sigma_{n}^{2}}}},} & {{Eq}\mspace{14mu}(6)}\end{matrix}$where M (Ŝ₁(k), Ŝ₂ (k)) denotes a log-likelihood associated with aparticular value of each code bit used to generate modulation symbols indesired transmission S₁(k) and interfering transmission S₂ (k). An LLRof a given code bit b used to generate the desired transmission S₁(k)may be expressed as follows:

$\begin{matrix}{{{LLR}_{b} = {{\max\limits_{{{\hat{S}}_{1}{(k)}},{{{{\hat{S}}_{2}{(k)}}:b} = 0}}{M\left( {{{\hat{S}}_{1}(k)},{{\hat{S}}_{2}(k)}} \right)}} - {\max\limits_{{{\hat{S}}_{1}{(k)}},{{{{\hat{S}}_{2}{(k)}}:b} = 1}}{M\left( {{{\hat{S}}_{1}(k)},{{\hat{S}}_{2}(k)}} \right)}}}},} & {{Eq}\mspace{14mu}(7)}\end{matrix}$where LLR_(b) is the LLR of code bit b.

UE 120 may perform joint demodulation based on maximum likelihood (ML)estimation or some other technique known in the art. UE 120 may performjoint demodulation with (i) channel estimates Ĥ₁ and Ĥ₂ for the servingand neighbor cells, respectively, (ii) a known modulation order of thedesired transmission, and (iii) a known or assumed modulation order ofthe interfering transmission. UE 120 may obtain demodulated symbolsŜ₁(k) and Ŝ₂(k) for both the desired and interfering transmissions fromthe joint demodulation. UE 120 may discard the demodulated symbols forthe interfering transmission and may process (e.g., decode) thedemodulated symbols for the desired transmission to recover the datasent by serving cell 110 to UE 120.

Regardless of the interference mitigation technique selected for use, UE120 may need to estimate the wireless channel from neighbor cell 112 toUE 120 in order to perform interference mitigation. UE 120 may performchannel estimation in different manners depending on how the referencesignal (or pilot) is transmitted.

In one design, a cell may transmit a common pilot to all UEs. The commonpilot may also be referred to as a cell-specific reference signal (CRS),etc. The cell may transmit the common pilot from each antenna at thecell without any precoding. The cell may transmit data to a UE with orwithout precoding and at a suitable transmit power level. The transmitpower level for data may be specified relative to the transmit powerlevel for pilot and may be given by a T2P value.

UE 120 may estimate the channel gain for each antenna of each cell ofinterest based on the common pilot transmitted from that cell on thatantenna. UE 120 may then obtain a channel estimate for each cell m, asfollows:H _(m) =G _(m1) ·W _(m1)·√{square root over (P _(m))}+ . . . +G _(mN) ·W_(mN)·√{square root over (P _(m))}.  Eq (8)where G_(m1) through G_(mN) are channel gains for N antennas at cell m,where N≥1,

W_(m1) through W_(nN) are precoding weights for the N antennas at cellm,

P_(m) is a power gain determined by the T2P of the interferingtransmission, and

H_(m) is the channel gain from cell m to UE 120.

As shown in equation (8), the actual channel observed by a datatransmission from cell m may depend on precoding weights for the Nantennas at cell m and the T2P used for the data transmission. Cell 112may send the precoding weights and T2P directly or indirectly torecipient UE 122. For example, cell 112 may send PMI identifying theprecoding weights in a downlink grant and may send the T2P via upperlayer signaling to UE 122. Cell 112 may also send the precoding weightsindirectly via a selected transmission mode, such as LD-CDD in LTE. UE122 may be able to determine the precoding weight based on the direct orindirect signaling from cell 112. UE 122 may then reconstruct the actualchannel based on estimated channel gains for the N antennas of cell 112and the transmission parameters (e.g., the PMI and/or T2P) signaled toUE 122. These parameters may be made available to interfered UE 120served by a different cell 110, so that UE 120 can perform interferencemitigation.

In another design, a cell may transmit a dedicated pilot to a specificUE being served. The dedicated pilot may also be referred to as adedicated reference signal (DRS), a UE-specific reference signal(UE-RS), etc. The cell may transmit the dedicated pilot on some of thedownlink resources used for data transmission and may process (e.g.,precode) the dedicated pilot in similar manner as for the datatransmission. The dedicated pilot would then observe the same overallchannel as the data transmission. The recipient UE may obtain a channelestimate for the cell based on the dedicated pilot, without having toknow the processing (e.g., precoding and power scaling) performed by thecell for the dedicated pilot and the data transmission. Similarly, aninterfered UE may also obtain a channel estimate for the cell based onthe dedicated pilot transmitted by the cell.

As noted above, pertinent transmission parameters for a served UE may besent to an interfered UE to enable to the interfered UE to performinterference mitigation. These parameters may include any of theparameters described above, e.g., the modulation order, MCS, T2P,precoding information, transmission rank, downlink resources, etc. Theseparameters may be sent in various manners.

In one design, a given cell m may send transmission parameters for otherUEs served by neighbor cells to UEs served by cell m. Cell m may sendtransmission parameters for its UEs to the neighbor cells (e.g., via thebackhaul, as shown in FIG. 1). Cell m may also receive transmissionparameters for other UEs served by the neighbor cells, e.g., via thebackhaul. Cell m and the neighbor cells may exchange transmissionparameters for certain UEs (e.g., UEs located near the edge of coverage)instead of all UEs. The cell-edge UEs may be identified based on thelocations of the UEs served by these cells and/or pilot measurementsmade by the UEs. Cell m may send the transmission parameters for theother UEs to its UEs, e.g., via broadcast signaling to all UEs, orunicast signaling to specific UEs, or multicast signaling to groups ofUEs. A UE served by cell m may receive transmission parameters for oneor more other UEs served by the neighbor cells and may performinterference mitigation based on the transmission parameters. Thisdesign may allow each cell to reliably send transmission parameters forother UEs in neighbor cells to the UEs served by that cell usingexisting signaling scheme.

In another design, a given cell m may send transmission parameters forits UEs to other UEs by neighbor cells. Cell m may send thesetransmission parameters via broadcast, unicast, or multicast signalingat a sufficiently high transmit power level to enable other UEs in theneighbor cells to reliably receive the transmission parameters. A UEserved by cell m may receive signaling comprising transmissionparameters for one or more other UEs served by one or more neighborcells. The UE may perform interference mitigation based on thetransmission parameters. This design may allow each cell to sendtransmission parameters for its UEs to other UEs in neighbor cellswithout having to exchange the parameters via the backhaul.

In general, transmission parameters for a given UE z may be sent by aserving cell, a neighbor cell, and/or some other entity to one or moreinterfered UEs. In one design, the sent transmission parameters may beactual parameters that are actually used for UE z. This design may allowthe interfered UEs to demodulate the data transmission to UE z. Inanother design, the sent transmission parameters may be defaultparameters that are likely to be used for UE z. For example, the systemmay support QPSK, 16-QAM and 64-QAM, and the sent transmissionparameters may convey a default modulation order of QPSK. In this case,the modulation order used for UE z may likely be QPSK but may also be16-QAM or 64-QAM. In this design, the interfered UEs may performdemodulation based on the default modulation order. The interfered UEsmay obtain the maximum gain when the actual modulation order is QPSK andmay obtain substantial gain even when the actual modulation order is16-QAM or 64-QAM. A cell may send different default values of a giventransmission parameter for different sets of resources (e.g., differentsets of frequency subbands, different subframes, or different sets oftime frequency blocks) that may be used for data transmission. The cellmay use the default parameter values for each set of resources, to theextent possible, in order to improve interference mitigation performanceby the interfered UEs.

In another aspect, a cell may send transmission parameters for a UE viaa dedicated pilot sent to the UE. The cell may generate the dedicatedpilot based on a PN sequence, or a scrambling sequence, or a constantamplitude zero auto correlation (CAZAC) sequence, or some othersequence. For clarity, the following description assumes the use of a PNsequence. The cell may encode or scramble the dedicated pilot based onthe transmission parameters, e.g., by generating the PN sequence basedon these parameters.

FIG. 2 shows a block diagram of a design of a transmit processor 200that can generate a dedicated pilot carrying transmission parameters fordata transmission for a UE. Transmit processor 200 may be part of a basestation/cell. Within transmit processor 200, an encoder 210 may receivedata for the UE being served, encode the data based on a selected codingscheme or code rate, and provide coded data. A symbol mapper 212 may mapthe coded data to modulation symbols based on a selected modulationorder. The modulation symbols for data may be referred to as datasymbols.

A PN generator 220 may receive a set of parameters for the UE beingserved and may generate a PN sequence based on this parameter set. Theparameter set may include a cell ID of the cell transmitting the dataand dedicated pilot, one or more transmission parameters for the datatransmission, and/or other parameters. The transmission parameters forthe data transmission may include the selected modulation order for thedata transmission and/or any of the parameters described above. In onedesign, PN generator 220 may generate a seed value based on theparameter set and may initialize a linear feedback shift register (LFSR)based on the seed value. The LFSR may then generate the PN sequencebased on a particular polynomial generator. A symbol mapper 222 may mapthe bits in the PN sequence to modulation symbols based on a modulationorder used for the dedicated pilot, which may be different from themodulation order used for data transmission. The modulation symbols forthe pilot may be referred to as pilot symbols.

A multiplexer (Mux) 230 may receive the data symbols from symbol mapper212 and the pilot symbols from symbol mapper 222. Multiplexer 230 mayprovide the data symbols to resources used for data transmission and mayprovide the pilot symbols to resources used for the dedicated pilot. Theresources for data transmission and the resources for the dedicatedpilot may be part of the resources allocated to the recipient UE fordata transmission.

As shown in FIG. 2, the dedicated pilot may carry a signature of adownlink grant for the recipient UE. The recipient UE can obtain thetransmission parameters from a downlink grant sent by the cell and canreadily generate the PN sequence for the dedicated pilot. An interferedUE may demodulate the dedicated pilot by evaluating different hypothesesfor the set of parameters sent in the dedicated pilot. For example, onlythe modulation order may be sent in the dedicated pilot, and only threemodulation orders of QPSK, 16-QAM and 64-QAM may be supported by thesystem. The interfered UE may then demodulate the dedicated pilot forthree hypotheses of QPSK, 16-QAM and 64-QAM and may obtain ademodulation metric for each hypothesis. The interfered UE may obtain achannel estimate as well as the modulation order of the datatransmission based on the hypothesis with the best demodulation metric.

For symbol-level interference mitigation, it may be desirable for aninterfered UE to be able to distinguish between modulation symbols of adesired transmission and modulation symbols of an interferingtransmission. The wireless channel for a serving cell may often bedifferent from the wireless channel for a neighbor cell. Hence, theinterfered UE can typically distinguish between the modulation symbolsof the desired and interfering transmissions after performing channelestimation. However, the distinction between the modulation symbols ofthe desired and interfering transmissions may be limited in certaincases, e.g., when the wireless channels for the serving cell and theneighbor cell are relatively close. Furthermore, uncertainty in channelknowledge (e.g., due to an unknown transmission parameter such as T2P)may lead to limited ability to distinguish between the modulationsymbols of the desired and interfering transmissions, especially in thepresence of noise.

In yet another aspect, scrambling may be performed at symbol level toenable an interfered UE to distinguish between modulation symbols ofdesired and interfering transmissions. A given cell may performsymbol-level scrambling by multiplying modulation symbols (or datasymbols) of a data transmission to a served UE with a scramblingsequence of modulation symbols (or scrambling symbols). The scramblingsequence may be specific for the cell (e.g., generated based on a cellID) and/or may be specific for the served UE (e.g., generated based on aUE ID). For other cell interference mitigation, it may be better to usecell-specific scrambling, so that the scrambling sequence may be knownto interfered UEs in neighbor cells. In any case, the scrambling symbolscan map the data symbols for the served UE to scrambled symbols that canbe distinguished from the data symbols for an interfered UE. In onedesign, the scrambling symbols may be generated based on a modulationorder/scheme that is different from the modulation orders available fordata transmission. In another design, the scrambling symbols may begenerated based on a modulation order that can map the data symbols forthe served UE so that the scrambled symbols of the interferingtransmission do not appear as valid scrambled symbols from the servingcell.

In one design, the scrambling symbols may be generated based on 8-PSK.This design may result in the scrambled symbols being rotated inquadrature as well as along diagonal axes. The scrambled symbolstransmitted by the serving cell (e.g., with the data symbols beinggenerated based on QPSK, 16-QAM, or 64-QAM) may then be readilydistinguished from the scrambled symbols transmitted by the interferingcell. In contrast, if the scrambling symbols are generated based on QPSKand the data symbols are generated based on QPSK, 16-QAM, or 64-QAM,then the scrambled symbols from the serving and interferingtransmissions may resemble each other. In general, the scrambled symbolsshould be defined by a signal constellation that does not resemble anysignal constellation for data symbols.

Symbol-level scrambling for data transmission may improve robustness ofinterference mitigation. Symbol-level scrambling is different fromscrambling on information bits provided to an encoder or code bitsgenerated by the encoder. Symbol-level scrambling may allow the datasymbols of the desired and interfering transmissions to be distinguishedeven if these transmissions undergo the same wireless channel.

FIG. 3 shows a block diagram of a design of a transmit processor 300that can perform symbol-level scrambling. Within transmit processor 300,an encoder 310 may receive data for a UE being served, encode the databased on a selected coding scheme or code rate, and provide coded data.A symbol mapper 312 may map the coded data to modulation symbols (ordata symbols) based on a selected modulation order. A PN generator 320may receive one or more scrambling parameters and may generate a PNsequence based on the scrambling parameter(s). The scramblingparameter(s) may include a cell ID for cell-specific scrambling, or a UEID of the served UE for UE-specific scrambling, or some other parameter,or a combination thereof. A symbol mapper 322 may map the bits in the PNsequence to modulation symbols (or scrambling symbols) based on amodulation order (e.g., 8-PSK) used for the scrambling sequence. Amultiplier 330 may receive the data symbols from symbol mapper 312 andthe scrambling symbols from symbol mapper 322. Multiplier 330 maymultiply each data symbol with a corresponding scrambling symbol togenerate a corresponding scrambled symbol.

The received signal at UE 120, when symbol-level scrambling is performedby each cell, may be expressed as:X(k)=H ₁ ·S ₁(k)·Q ₁(k)+H ₂ ·S ₂(k)·Q ₂(k)+N,  Eq (9)where Q₁(k) is a scrambling sequence for the desired transmission, and

Q₂(k) is a scrambling sequence for the interfering transmission.

UE 120 may descramble the received signal based on the scramblingsequence for the desired transmission to obtain a descrambled signal,which may be expressed as:Z(k)=X(k)·Q ₁*(k)=H ₁ ·S ₁(k)+H ₂ ·S ₂(k)·Q ₂(k)·Q ₁*(k)+N·Q ₁*(k),  Eq(10)where Z(k) is a descrambled signal for serving cell 110.

As shown in equation (10), the descrambled signal includes a desiredtransmission corresponding to H₁·S₁(k) as well as a scrambledinterfering transmission corresponding to H₂·S₂(k)·Q₂(k)·Q₁*(k). Thedesired transmission may comprise data symbols for a selected modulationorder. The scrambled interfering transmission may comprise scrambledsymbols, which may be rotated by a scrambling sequence Q₂(k)·Q₁*(k) andmay thus be distinguishable from the data symbols.

FIG. 4 shows a block diagram of a design of a receive processor 400 thatcan perform symbol-level descrambling. Receive processor 400 may be partof a UE. Within receive processor 400, a received signal comprisingreceived symbols may be provided to a multiplier 412. A unit 414 mayreceive a scrambling sequence for a desired transmission and may providea conjugated scrambling sequence as a descrambling sequence. Multiplier412 may multiply each received symbol with a corresponding symbol in thedescrambling sequence and provide a corresponding descrambled symbol. Ademodulator 420 may perform demodulation with symbol-level interferencemitigation based on channel estimates for the serving cell and one ormore interfering cells as well as one or more transmission parameterssuch as modulation order, T2P, etc. Demodulator 420 may performdemodulation with soft-symbol interference cancellation as shown inequations (3) to (5) or may perform joint demodulation based on thecriterion shown in equation (6). In either case, demodulator 420 mayprovide demodulated symbols for the serving cell. A decoder 430 mayreceive and decode the demodulated symbols and provide decoded data forUE 120.

UE 120 may perform demodulation and decoding for a desired transmissionfrom serving cell 110 in various manners. In a first design, UE 120 mayfirst perform demodulation and decoding without interference mitigationto recover data sent by serving cell 110. If decoding is unsuccessful,then UE 120 may next perform demodulation and decoding with interferencemitigation to recover the data sent by serving cell 110. In a seconddesign, UE 120 may perform demodulation and decoding with interferencemitigation (and may not attempt to perform demodulation and decodingwithout interference mitigation) to recover the data sent by servingcell 110. In a third design, UE 120 may perform demodulation anddecoding using either the first design or the second design based on oneor more factors such as the channel quality for serving cell 110. Forexample, UE 120 may perform demodulation and decoding using the firstdesign if the channel quality is sufficiently good and using the seconddesign otherwise.

In another design, UE 120 may perform demodulation for the desiredtransmission by treating an interfering transmission as interference,e.g., as shown in equation (2), and may obtain a metric for thisdemodulation. UE 120 may also perform demodulation for the desiredtransmission with interference mitigation based on assumed transmissionparameters for the interfering transmission and may obtain a metric forthis demodulation. UE 120 may then select the demodulation output withthe better metric.

UE 120 may perform demodulation and decoding with interferencemitigation in various manners. In one design, UE 120 may estimate andcancel the interference from all interfering transmissions to obtain aninterference-canceled signal and may then decode theinterference-canceled signal to recover data sent by serving cell 110 toUE 120. In another design, UE 120 may estimate and cancel theinterference from one interfering transmission at a time (e.g., startingwith the strongest interfering transmission) and may decode theinterference-canceled signal, after canceling the interference from eachinterfering transmission, to recover the data sent by serving cell 110to UE 120. In yet another design, UE 120 may estimate and cancel theinterference from one set of interfering transmissions at a time and maydecode the interference-canceled signal after canceling the interferencefrom each set of interfering transmissions. UE 120 may also performdemodulation and decoding with interference mitigation in other manners.

UE 120 may perform interference mitigation in various manners. In onedesign, UE 120 may perform interference mitigation for each interferingtransmission based on known or assumed transmission parameters for thatinterfering transmission. In another design, UE 120 may perform blinddemodulation for an interfering transmission based on differenthypotheses for transmission parameters used for the interferingtransmission. This design may be used if the transmission parameters (i)are not sent by any cell or (ii) are sent but not received by theinterfered UE for any reason. The interfered UE may perform demodulationfor the interfering transmission based on each hypothesis and may obtaina metric for the hypothesis. For example, the interfered UE may performdemodulation based on a hypothesis of QPSK, and another hypothesis of16-QAM, and yet another hypothesis of 64-QAM. The interfered UE may thenselect the hypothesis with the best metric and may decode thedemodulated symbols associated with the selected hypothesis.

For clarity, various techniques for interference mitigation have beendescribed specifically for data transmission on the downlink. Some orall of these techniques may also be used for interference mitigation fordata transmission on the uplink. A cell may perform interferencemitigation as described above to mitigate interference due tointerfering transmissions from UEs served by neighbor cells.

The techniques described herein may be used for interference mitigationfor interfering transmissions from neighbor cells, which may be referredto as inter-cell interference mitigation. The techniques may also beused for intra-cell interference mitigation for multi-user MIMO(MU-MIMO). For MU-MIMO, multiple UEs may be scheduled for datatransmission on the same time-frequency resources with spatialseparation (e.g., via beamforming) at a cell. MU-MIMO is typicallydesigned to be transparent to each UE being scheduled. Hence, a given UEmay not be aware of other UE(s) being scheduled concurrently on the sametime-frequency resources. The UE may have limited information about theother data transmission(s) sent by the cell to the other UE(s). Forexample, the UE may not know the presence of the other UE(s), thetransmission rank, MCS, resources, etc. Interference mitigation forMU-MIMO (or intra-cell interference mitigation) may be performed insimilar manner as inter-cell interference mitigation. The maindifferences between intra-cell interference mitigation and inter-cellinterference mitigation may be as follows:

-   -   Inter-cell interference mitigation may require communication        between cells to convey transmission parameters whereas        intra-cell interference mitigation may be localized to a single        cell and may not require inter-cell communication; and    -   Any cell-specific aspect in the inter-cell context may be        localized to a single cell in the intra-cell context. For        example, cell ID in cell-specific scrambling (e.g., for UE-RS        scrambling and/or data scrambling to differentiate between the        serving and interfering cells) may be replaced with multiple        cell IDs for the same cell and assigned (e.g., semi-statically        or dynamically) to different UEs being scheduled concurrently        for MU-MIMO.

A UE may perform interference mitigation for MU-MIMO in similar manneras an interfered UE performing interference mitigation for aninterfering transmission from a neighbor cell. The UE may receiveinformation for at least one transmission parameter for one or moreother UEs scheduled concurrently with the UE for a MU-MIMO transmission.The UE may perform packet-level or symbol-level interference mitigationfor each co-scheduled UE, as described above. The UE may also performsoft-symbol interference cancellation or joint demodulation for eachco-scheduled UE, as also described above.

FIG. 5 shows a design of a process 500 for performing interferencemitigation. Process 500 may be performed by a first UE for datatransmission on the downlink (as described below), or by a base stationfor data transmission on the uplink, or by some other entity. The firstUE may obtain a received signal comprising a first data transmissionsent by a first cell to the first UE and a second data transmission sentby a second cell to a second UE (block 512). The first UE may alsoobtain information for at least one transmission parameter for thesecond data transmission (block 514). The first UE may performinterference mitigation for the second data transmission, based on theinformation for the at least one transmission parameter, to recover thefirst data transmission sent to the first UE (block 516).

In one design of block 516, the first UE may perform interferencemitigation for the second data transmission at the packet level by (i)decoding the second data transmission to recover at least one packetsent by the second cell to the second UE, (ii) estimating interferencedue to the second data transmission based on the at least one packet,and (iii) canceling the estimated interference. In another design, thefirst UE may perform interference mitigation for the second datatransmission at the symbol level, without decoding the second datatransmission to recover any packet sent by the second cell to the secondUE.

In one design of symbol-level interference mitigation, the first UE mayperform soft-symbol interference cancellation. The first UE may firstdemodulate the received signal based on the information for the at leastone transmission parameter to obtain demodulated symbols for the seconddata transmission. The first UE may then estimate interference due tothe second data transmission based on the demodulated symbols for thesecond data transmission. The first UE may subtract the estimatedinterference from the received signal to obtain an interference-canceledsignal. The first UE may then demodulate the interference-canceledsignal to obtain demodulated symbols for the first data transmission.

In another design of symbol-level interference mitigation, the first UEmay perform joint demodulation on the received signal based on theinformation for the at least one transmission parameter to obtaindemodulated symbols for both the first and second data transmissions.The first UE may discard the demodulated symbols for the second datatransmission and may process (e.g., decode) the demodulated symbols forthe first data transmission.

In yet another design of symbol-level interference mitigation, the firstUE may perform iterative demodulation and decoding. The first UE mayperform demodulation with interference mitigation and decoding for aplurality of iterations. The output of demodulation in each iterationmay be used for decoding in the same iteration. The output of decodingin each iteration, except for the final iteration, may be used fordemodulation in the next iteration.

In one design, the first UE may perform demodulation with interferencemitigation for a plurality of hypotheses. Each hypothesis may correspondto a different set of one or more values for one or more transmissionparameters. The first UE may select the demodulation output for thehypothesis with the best metric. In another design, the first UE mayperform demodulation with interference mitigation for a singlehypothesis, e.g., based on known or assumed value for the at least onetransmission parameter.

In one design, the first UE may obtain a channel estimate for a wirelesschannel from the second cell to the first UE based on information forone or more transmission parameters, e.g., T2P, precoding information,transmission rank, etc. This design may be used if the second celltransmits a common pilot (or a cell-specific reference signal) for allUEs served by the second cell. This design may also be used if thesecond cell transmits a dedicated pilot (or a UE-specific referencesignal) to the second UE. In any case, the first UE may performdemodulation with interference mitigation based on the channel estimateto obtain demodulated symbols for the first data transmission.

In one design, the first UE may obtain the information for the at leastone transmission parameter from signaling transmitted by the first cellor the second cell. In another design, the first UE may obtain theinformation for the at least one transmission parameter from a pilot(e.g., a dedicated pilot) transmitted by the second cell to the secondUE. The first UE may also obtain the information for the at least onetransmission parameter in other manners. The information for the atleast one transmission parameter may comprise a modulation order, or anMCS, or a T2P, or precoding information (e.g., PMI, transmission modeindicator, etc.), or a transmission rank, or assigned resources for thesecond data transmission, or some other information, or a combinationthereof. In one design, the first UE may obtain actual value of the atleast one transmission parameter used by the second cell for the seconddata transmission. In another design, the first UE may obtain defaultvalue of the at least one transmission parameter likely to be used bythe second cell for the second data transmission.

FIG. 6 shows a design of an apparatus 600 for performing interferencemitigation. Apparatus 600 includes a module 612 to obtain at a first UEa received signal comprising a first data transmission sent by a firstcell to the first UE and a second data transmission sent by a secondcell to a second UE, a module 614 to obtain information for at least onetransmission parameter for the second data transmission, and a module616 to perform interference mitigation for the second data transmission,based on the information for the at least one transmission parameter, torecover the first data transmission sent to the first UE.

FIG. 7 shows a design of a process 700 for sending transmissionparameters to support interference mitigation. Process 700 may beperformed by a cell (as described below) or by some other entity.Information for at least one transmission parameter for a datatransmission sent by a first cell to a UE may be obtained (block 712).The information for the at least one transmission parameter may betransmitted to at least one UE served by a second cell to enable the atleast one UE to perform interference mitigation for the datatransmission sent by the first cell to the UE (block 714).

In one design, blocks 712 and 714 may be performed by the first cell. Inanother design, blocks 712 and 714 may be performed by the second cell,which may receive the information for the at least one transmissionparameter from the first cell via the backhaul. In any case, theinformation for the at least one transmission parameter may comprise amodulation order, or an MCS, or a T2P, or precoding information, or atransmission rank, or assigned resources for the data transmission, orsome other information, or a combination thereof.

In one design, information for transmission parameters for UEs or cellsthat are potential interferers may be sent to other UEs for interferencemitigation. The UE or the first cell may be identified as potentialinterferer to the at least one UE based on pilot measurement from theUE, or the location of the UE, or pilot measurements from the at leastone UE, or the location of the at least one UE, or some otherinformation, or a combination thereof.

FIG. 8 shows a design of an apparatus 800 for sending transmissionparameters to support interference mitigation. Apparatus 800 includes amodule 812 to obtain information for at least one transmission parameterfor a data transmission sent by a first cell to a UE, and a module 814to transmit the information for the at least one transmission parameterto at least one UE served by a second cell to enable the at least one UEto perform interference mitigation for the data transmission sent by thefirst cell to the UE.

FIG. 9 shows a design of a process 900 for sending transmissionparameters via a pilot. Process 900 may be performed by a cell (asdescribed below) or by some other entity. The cell may generate a datatransmission based on at least one transmission parameter (block 912).The cell may also generate a pilot (e.g., a dedicated pilot) comprisinginformation for the at least one transmission parameter, which mayinclude any of the information listed above (block 914). In one design,the cell may generate a PN sequence based on the information for the atleast one transmission parameter and may then generate modulationsymbols for the pilot based on the PN sequence. The cell may transmitthe pilot and the data transmission to a recipient UE (block 916). OtherUEs may use the information for the at least one transmission parameterin the pilot to perform interference mitigation for the datatransmission.

FIG. 10 shows a design of an apparatus 1000 for sending transmissionparameters via a pilot. Apparatus 1000 includes a module 1012 togenerate a data transmission based on at least one transmissionparameter, a module 1014 to generate a pilot comprising information forthe at least one transmission parameter, and a module 1016 to transmitthe pilot and the data transmission.

FIG. 11 shows a design of a process 1100 for performing symbol levelscrambling. Process 1100 may be performed by a cell (as described below)or by some other entity. The cell may generate modulation symbols for adata transmission (block 1112). The cell may scramble the modulationsymbols based on a scrambling sequence to obtain scrambled symbols(block 1114). The cell may transmit the scrambled symbols for the datatransmission (block 1116).

In one design, the cell may generate the scrambling sequence based on acell ID, or a UE ID, or both. In one design, the cell may generate thescrambling sequence based on (i) a modulation order that is not used forthe data transmission, or (ii) a modulation order that does not map themodulation symbols for the data transmission to other valid modulationsymbols, or (iii) 8-PSK or some other suitable modulation order.

FIG. 12 shows a design of an apparatus 1200 for performing symbol levelscrambling. Apparatus 1200 includes a module 1212 to generate modulationsymbols for a data transmission, a module 1214 to scramble themodulation symbols based on a scrambling sequence to obtain scrambledsymbols, and a module 1216 to transmit the scrambled symbols for thedata transmission.

FIG. 13 shows a design of a process 1300 for performing symbol leveldescrambling. Process 1300 may be performed by a UE (as described below)or by some other entity. The UE may descramble received symbols based ona scrambling sequence to obtain descrambled symbols (block 1312). In onedesign, the UE may generate the scrambling sequence based on a cell ID,or a UE ID, or both. The UE may demodulate the descrambled symbols toobtain demodulated symbols (block 1314).

In one design, the UE may obtain the received symbols comprising a firstdata transmission sent by a first cell to the UE and a second datatransmission sent by a second cell to a second UE. The UE may alsoobtain information for at least one transmission parameter for thesecond data transmission. The UE may demodulate the descrambled symbolswith interference mitigation based on the information for at least onetransmission parameter to recover the first data transmission sent tothe UE.

FIG. 14 shows a design of an apparatus 1400 for performing symbol leveldescrambling. Apparatus 1400 includes a module 1412 to descramblereceived symbols based on a scrambling sequence to obtain descrambledsymbols, and a module 1414 to demodulate the descrambled symbols (e.g.,with interference mitigation) to obtain demodulated symbols.

The modules in FIGS. 6, 8, 10, 12 and 14 may comprise processors,electronic devices, hardware devices, electronic components, logicalcircuits, memories, software codes, firmware codes, etc., or anycombination thereof.

FIG. 15 shows a block diagram of a design of base station 110 and UE 120in FIG. 1. Base station 110 may be equipped with T antennas 1534 athrough 1534 t, and UE 120 may be equipped with R antennas 1552 athrough 1552 r, where in general T≥1 and R≥1.

At base station 110, a transmit processor 1520 may receive data from adata source 1512 for one or more UEs, process (e.g., encode andmodulate) the data for each UE based on one or more MCSs selected forthat UE, and provide data symbols for all UEs. Transmit processor 1520may also receive control information (e.g., grants, transmissionparameters for UEs communicating with neighbor cells, etc.) from acontroller/processor 1540. Processor 1520 may process the controlinformation and provide control symbols. Processor 1520 may alsogenerate pilot/reference symbols for one or more pilots/referencesignals, e.g., dedicated pilot, common pilot, etc. A transmit (TX) MIMOprocessor 1530 may perform spatial processing (e.g., precoding) on thedata symbols, the control symbols, and/or the pilot symbols, ifapplicable, and may provide T output symbol streams to T transmitters(TMTRs) 1532 a through 1532 t. Each transmitter 1532 may process arespective output symbol stream (e.g., for OFDM, etc.) to obtain anoutput sample stream. Each transmitter 1532 may further process (e.g.,convert to analog, amplify, filter, and upconvert) the output samplestream to obtain a downlink signal. T downlink signals from transmitters1532 a through 1532 t may be transmitted via T antennas 1534 a through1534 t, respectively.

At UE 120, antennas 1552 a through 1552 r may receive the downlinksignals from base station 110 and may provide received signals toreceivers (RCVRs) 1554 a through 1554 r, respectively. Each receiver1554 may condition (e.g., filter, amplify, downconvert, and digitize)its received signal to obtain input samples. Each receiver 1554 mayfurther process the input samples (e.g., for OFDM, etc.) to obtainreceived symbols. A MIMO detector 1556 may obtain received symbols fromall R receivers 1554 a through 1554 r, perform MIMO detection on thereceived symbols if applicable, and provide detected symbols. A receiveprocessor 1558 may process (e.g., demodulate with interferencemitigation and decode) the detected symbols, provide decoded data for UE120 to a data sink 1560, and provide decoded control information to acontroller/processor 1580.

On the uplink, at UE 120, a transmit processor 1564 may receive andprocess data from a data source 1562 and control information fromcontroller/processor 1580. Processor 1564 may also generatepilot/reference symbols for pilot/reference signal. The symbols fromtransmit processor 1564 may be precoded by a TX MIMO processor 1566 ifapplicable, further processed by transmitters 1554 a through 1554 r(e.g., for SC-FDM, OFDM, etc.), and transmitted to base station 110. Atbase station 110, the uplink signals from UE 120 may be received byantennas 1534, processed by receivers 1532, detected by a MIMO detector1536 if applicable, and further processed by a receive processor 1538 toobtain decoded data and control information sent by UE 120. Processor1538 may provide the decoded data to a data sink 1539 and the decodedcontrol information to controller/processor 1540.

Transmit processors 1520 and 1564 may each implement transmit processor200 in FIG. 2, transmit processor 300 in FIG. 3, and/or other transmitprocessor designs. Receive processors 1538 and 1558 may each implementreceive processor 400 in FIG. 4 and/or other receive processor designs.

Controllers/processors 1540 and 1580 may direct the operation at basestation 110 and UE 120, respectively. Processor 1540 and/or otherprocessors and modules at base station 110 may perform or direct process700 in FIG. 7, process 900 in FIG. 9, process 1100 in FIG. 11, and/orother processes for the techniques described herein. Processor 1580and/or other processors and modules at UE 120 may perform or directprocess 500 in FIG. 5, process 1300 in FIG. 13, and/or other processesfor the techniques described herein. Memories 1542 and 1582 may storedata and program codes for base station 110 and UE 120, respectively. Ascheduler 1544 may schedule UEs for data transmission on the downlinkand/or uplink.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the disclosure herein may be implemented as electronichardware, computer software, or combinations of both. To clearlyillustrate this interchangeability of hardware and software, variousillustrative components, blocks, modules, circuits, and steps have beendescribed above generally in terms of their functionality. Whether suchfunctionality is implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem. Skilled artisans may implement the described functionality invarying ways for each particular application, but such implementationdecisions should not be interpreted as causing a departure from thescope of the present disclosure.

The various illustrative logical blocks, modules, and circuits describedin connection with the disclosure herein may be implemented or performedwith a general-purpose processor, a digital signal processor (DSP), anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA) or other programmable logic device, discrete gate ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. Ageneral-purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with thedisclosure herein may be embodied directly in hardware, in a softwaremodule executed by a processor, or in a combination of the two. Asoftware module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such that theprocessor can read information from, and write information to, thestorage medium. In the alternative, the storage medium may be integralto the processor. The processor and the storage medium may reside in anASIC. The ASIC may reside in a user terminal. In the alternative, theprocessor and the storage medium may reside as discrete components in auser terminal.

In one or more exemplary designs, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples and designs described herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

What is claimed is:
 1. A method for wireless communication, wherein themethod is performed at a first user equipment, the method comprising:obtaining at the first user equipment a received signal comprising afirst data transmission sent by a first cell to the first user equipmentand a second data transmission sent by a second cell to a second userequipment; obtaining, from the second data transmission sent by thesecond cell, information for at least one transmission parameter for thesecond data transmission; and performing symbol-level interferencemitigation for the second data transmission, based on the informationfor the at least one transmission parameter, to recover the first datatransmission sent to the first user equipment, wherein the performingsymbol-level interference mitigation comprises: demodulating symbols forthe second data transmission without decoding the second datatransmission to recover any packet sent by the second cell to the seconduser equipment, demodulating the received signal based on theinformation for the at least one transmission parameter to obtaindemodulated symbols for the second data transmission, estimatinginterference due to the second data transmission based on thedemodulated symbols for the second data transmission, subtracting theestimated interference from the received signal to obtain aninterference-canceled signal, and demodulating the interference-canceledsignal to obtain demodulated symbols for the first data transmission. 2.The method of claim 1, wherein the performing symbol-level interferencemitigation further comprises performing joint demodulation on thereceived signal based on the information for the at least onetransmission parameter to obtain demodulated symbols for the first andsecond data transmissions.
 3. The method of claim 1, wherein theperforming symbol-level interference mitigation further comprisesperforming demodulation with symbol-level interference mitigation for aplurality of hypotheses, each hypothesis corresponding to a differentset of one or more values for one or more of the at least onetransmission parameter.
 4. The method of claim 1, wherein the performingsymbol-level interference mitigation further comprises obtaining achannel estimate for a wireless channel from the second cell to thefirst user equipment based on information for one or more of the atleast one transmission parameter, and performing demodulation withsymbol-level interference mitigation based further on the channelestimate to obtain the demodulated symbols for the first datatransmission.
 5. The method of claim 1, wherein the information for theat least one transmission parameter comprises at least one of: amodulation order, a modulation and coding scheme, a traffic-to-pilotratio, precoding information, a transmission rank, or assigned resourcesfor the second data transmission.
 6. The method of claim 1, wherein theobtaining the information for the at least one transmission parametercomprises obtaining actual value of the at least one transmissionparameter used by the second cell for the second data transmission. 7.The method of claim 1, wherein the obtaining the information for the atleast one transmission parameter comprises obtaining a default value ofthe at least one transmission parameter likely to be used by the secondcell for the second data transmission.
 8. The method of claim 1, whereinthe obtaining the information for the at least one transmissionparameter comprises obtaining the information for the at least onetransmission parameter from a pilot transmitted by the second cell tothe second user equipment.
 9. The method of claim 1, wherein thesymbol-level interference mitigation is based in part on a scramblingsequence.
 10. A first user equipment for wireless communication,comprising: means for obtaining at the first user equipment a receivedsignal comprising a first data transmission sent by a first cell to thefirst user equipment and a second data transmission sent by a secondcell to a second user equipment; means for obtaining, from the seconddata transmission sent by the second cell, information for at least onetransmission parameter for the second data transmission; and means forperforming symbol-level interference mitigation for the second datatransmission, based on the information for the at least one transmissionparameter, to recover the first data transmission sent to the first userequipment, wherein the means for performing symbol-level interferencemitigation comprises: means for demodulating symbols for the second datatransmission without decoding the second data transmission to recoverany packet sent by the second cell to the second user equipment, meansfor demodulating the received signal based on the information for the atleast one transmission parameter to obtain demodulated symbols for thesecond data transmission, means for estimating interference due to thesecond data transmission based on the demodulated symbols for the seconddata transmission, means for subtracting the estimated interference fromthe received signal to obtain an interference-canceled signal, and meansfor demodulating the interference-canceled signal to obtain demodulatedsymbols for the first data transmission.
 11. The first user equipment ofclaim 10, wherein the means for performing symbol-level interferencemitigation further comprises means for performing joint demodulation onthe received signal based on the information for the at least onetransmission parameter to obtain demodulated symbols for the first andsecond data transmissions.
 12. The first user equipment for wirelesscommunication of claim 10, wherein the symbol-level interferencemitigation is based in part on a scrambling sequence.
 13. A method forwireless communication, comprising: obtaining information for at leastone transmission parameter for a first data transmission sent by a firstcell to a user equipment; and transmitting, from the first cell, theinformation for the at least one transmission parameter to at least oneuser equipment served by a second cell to enable the at least one userequipment to perform symbol-level interference mitigation for the firstdata transmission sent by the first cell to the user equipment based onthe at least one transmission parameter, wherein the symbol-levelinterference mitigation comprises: demodulating symbols in the firstdata transmission from the first cell without decoding the first datatransmission from the first cell to recover any packet sent by the firstcell to the user equipment, demodulating a signal received from thesecond cell based on the information for the at least one transmissionparameter to obtain demodulated symbols for the first data transmission,estimating interference due to the first data transmission based on thedemodulated symbols for the first data transmission, subtracting theestimated interference from the received signal to obtain aninterference-canceled signal, and demodulating the interference-canceledsignal to obtain demodulated symbols for a second data transmission fromthe second cell.
 14. The method of claim 13, wherein the information forthe at least one transmission parameter comprises at least one of: amodulation order, a modulation and coding scheme, a traffic-to-pilotratio, precoding information, a transmission rank, or assigned resourcesfor the first data transmission.
 15. The method of claim 13, wherein theobtaining the information is performed by the first cell.
 16. The methodof claim 13, further comprising: identifying the user equipment or thefirst cell as potential interferer to the at least one user equipmentbased on at least one of: pilot measurement from the user equipment,location of the user equipment, pilot measurements from the at least oneuser equipment, or location of the at least one user equipment.
 17. Anapparatus for wireless communication, comprising: means for obtaininginformation for at least one transmission parameter for a first datatransmission sent by a first cell to a user equipment; and means fortransmitting, from the first cell, the information for the at least onetransmission parameter to at least one user equipment served by a secondcell to enable the at least one user equipment to perform symbol-levelinterference mitigation for the first data transmission sent by thefirst cell to the user equipment based on the at least one transmissionparameter, wherein the symbol-level interference mitigation comprises:demodulating symbols in the first data transmission from the first cellwithout decoding the first data transmission from the first cell torecover any packet sent by the first cell to the user equipment,demodulating a signal received from the second cell based on theinformation for the at least one transmission parameter to obtaindemodulated symbols for the first data transmission, estimatinginterference due to the first data transmission based on the demodulatedsymbols for the first data transmission, subtracting the estimatedinterference from the received signal to obtain an interference-canceledsignal, and demodulating the interference-canceled signal to obtaindemodulated symbols for a second data transmission from the second cell.18. The apparatus of claim 17, wherein the information for the at leastone transmission parameter comprises at least one of: a modulationorder, a modulation and coding scheme, a traffic-to-pilot ratio,precoding information, a transmission rank, or assigned resources forthe first data transmission.
 19. A first user equipment, comprising atleast one processor configured to: obtain a received signal comprising afirst data transmission sent by a first cell to the first user equipmentand a second data transmission sent by a second cell to a second userequipment; obtain, from the second data transmission sent by the secondcell, information for at least one transmission parameter for the seconddata transmission; and perform symbol-level interference mitigation forthe second data transmission, based on the information for the at leastone transmission parameter, to recover the first data transmission sentto the first user equipment, wherein performing symbol-levelinterference mitigation comprises: demodulating symbols for the seconddata transmission without decoding the second data transmission torecover any packet sent by the second cell to the second user equipment,demodulate the received signal based on the information for the at leastone transmission parameter to obtain demodulated symbols for the seconddata transmission, estimate interference due to the second datatransmission based on the demodulated symbols for the second datatransmission, subtract the estimated interference from the receivedsignal to obtain an interference-canceled signal, and demodulate theinterference-canceled signal to obtain demodulated symbols for the firstdata transmission.
 20. The first user equipment of claim 19, wherein thesymbol-level interference mitigation is based in part on a scramblingsequence.
 21. A non-transitory computer-readable medium comprising: codefor causing a first user equipment to obtain a received signalcomprising a first data transmission sent by a first cell to the firstuser equipment and a second data transmission sent by a second cell to asecond user equipment; code for causing the first user equipment toobtain, from the second data transmission sent by the second cell,information for at least one transmission parameter for the second datatransmission; and code for causing the first user equipment to obtainperform symbol-level interference mitigation for the second datatransmission, based on the information for the at least one transmissionparameter, to recover the first data transmission sent to the first userequipment, wherein the code for causing the first user equipment toperform symbol-level interference mitigation comprises code for causingthe first user equipment to: demodulate symbols for the second datatransmission without decoding the second data transmission to recoverany packet sent by the second cell to the second user equipment,demodulate the received signal based on the information for the at leastone transmission parameter to obtain demodulated symbols for the seconddata transmission, estimate interference due to the second datatransmission based on the demodulated symbols for the second datatransmission, subtract the estimated interference from the receivedsignal to obtain an interference-canceled signal, and demodulate theinterference-canceled signal to obtain demodulated symbols for the firstdata transmission.
 22. A first cell, comprising at least one processorconfigured to: obtain information for at least one transmissionparameter for a first data transmission sent by the first cell to a userequipment; and transmit, from the first cell, the information for the atleast one transmission parameter to at least one user equipment servedby a second cell to enable the at least one user equipment to performsymbol-level interference mitigation for the first data transmissionsent by the first cell to the user equipment based on the at least onetransmission parameter, wherein the symbol-level interference mitigationcomprises: demodulating symbols in the first data transmission from thefirst cell without decoding the first data transmission from the firstcell to recover any packet sent by the first cell to the user equipment,demodulating a signal received from the second cell based on theinformation for the at least one transmission parameter to obtaindemodulated symbols for the first data transmission, estimatinginterference due to the first data transmission based on the demodulatedsymbols for the first data transmission, subtracting the estimatedinterference from the received signal to obtain an interference-canceledsignal, and demodulating the interference-canceled signal to obtaindemodulated symbols for a second data transmission from the second cell.23. A non-transitory computer-readable medium comprising: code forcausing a first cell to obtaining information for at least onetransmission parameter for a first data transmission sent by the firstcell to a user equipment; and code for causing the first cell totransmit the information for the at least one transmission parameter toat least one user equipment served by a second cell to enable the atleast one user equipment to perform symbol-level interference mitigationfor the first data transmission sent by the first cell to the userequipment based on the at least one transmission parameter, wherein thesymbol-level interference mitigation comprises: demodulating symbols inthe first data transmission from the first cell without decoding thefirst data transmission from the first cell to recover any packet sentby the first cell to the user equipment, demodulating a signal receivedfrom the second cell based on the information for the at least onetransmission parameter to obtain demodulated symbols for the first datatransmission, estimating interference due to the first data transmissionbased on the demodulated symbols for the first data transmission,subtracting the estimated interference from the received signal toobtain an interference-canceled signal, and demodulating theinterference-canceled signal to obtain demodulated symbols for a seconddata transmission from the second cell.